The subject matter of the present invention is a new process for the preparation of derivatives of vinylcyclopropanecarboxylic acid as well as new derivatives of tetrahydro-alpha-pyrone which are to be used as starting products in this process. Additional subject matter is a process for the preparation of these new derivatives of tetrahydro-alpha-pyrone.
The vinylcyclopropanecarboxylic acid derivatives of formula I below are important intermediates for the production of insecticides in the pyrethroid class. The potency of these pyrethroids depends on, among other things, the stereochemical configuration of the active molecule. Thus, for example, the cis form of cypermethrin is more effective than the trans form. The syntheses of cypermethrin starts out from 2,2-dimethyl-3-(.beta.,.beta.-dichlorovinyl)-cyclopropane-1-carboxylic acid esters, which will be referred to hereinafter as cyclopropanecarboxylic acid esters, for the sake of simplicity (Synthetic Pyrethroids, ACS Symposium Series 42, p. 45). In European Patent B1 0 003 683 there is described a method of preparation, the purpose of which is the synthesis of the cis form of the above-mentioned cyclopropanecarboxylic acid ester. This method is here outlined briefly: ##STR1##
The disadvantage of this process is that the reaction to the cis form requires very special solvent mixtures, and that the preparation of the starting product can be accomplished only with great technical difficulty.
The problem therefore existed of finding a process for the preparation of the cyclopropane acid ester of formula I from easily obtained starting products different from those used heretofore, and which resulted in a very high content of cis isomers.